Police arrested three masked men after they allegedly broke off from an Occupy Boston march and invaded a packed fast-food restaurant in the Back Bay Saturday evening. Wearing facemasks and dark clothing and carrying homemade signs, the trio burst through the front doors of a Burger King, police said. Officers said that a full house of employees and customers “appeared to cower in fear and had a look of shock and disbelief on their faces at such a commotion.’’